The invention relates to a process for the preparation of microencapsulated polymers of high surface quality by polymerization of monomers enclosed in microcapsules, which leads to polymers having a soluble fraction of less than 6% and a swelling index between 1.3 and 9.
Ion exchangers having as uniform a particle size as possible (referred to below as "monodisperse") have recently become increasingly important because economic advantages can be obtained in many applications due to the more advantageous hydrodynamic properties of an exchanger bed of monodisperse ion exchangers. Monodisperse ion exchangers can be obtained by functionalizing monodisperse bead polymers. One possible method for preparing monodisperse bead polymers comprises producing monodisperse monomer droplets by spraying monomers into a continuous phase and then curing the droplets by polymerization. The formation of uniform droplet sizes can be supported by excitation of vibrations. For example, European Patent Application 51,210 describes a process for the preparation of spherical monomer droplets having a uniform particle size by excitation of vibrations in a laminar monomer stream. If it is intended to retain the monodispersity of the monomer droplets in the polymerization, coalescence and formation of new droplets must be excluded. A particularly effective method for preventing coalescence and formation of new droplets comprises the microencapsulation of the droplets according to European Patent Application 46,535.
A further possible method of preparing monodisperse bead polymers is the so-called seed/feed process, in which a monodispersed polymer ("seed") is swollen in the monomer and the latter is then polymerized. Seed/feed-processes are described, for example, in European Patent Applications 98,130 and 101,943. One possible method for particularly exact transfer of the particle size distribution of the seed polymer to the bead polymer is described in German Patent Application 19,634,393. Here, a microencapsulated polymer is used as a seed.
Microencapsulated seed polymers should have a high swelling index so that they can absorb a large amount of the added monomer in the seed/feed process. The swelling index ("SI") is defined as the quotient of the volume of the swollen polymer and the volume of the polymer which has not swollen. The swelling index can be controlled in a known manner by the content of crosslinking agents: low contents of crosslinking agents lead to high swelling indices and vice versa. Thus, for example, styrene polymers which are crosslinked with 0.2 to 2.0% by weight of divinylbenzene have swelling indices of 2.5 to 10 in toluene. However, seed polymers having a low degree of crosslinking have a very high fraction of uncrosslinked, soluble polymers. This fraction of uncrosslinked, soluble polymers in the seed polymer is undesired in many respects:
1. The polymerization of the swollen seed may be impaired by the fact that the polymer fractions dissolved from the seed by the added monomer cause the particles to adhere to one another. PA1 2. The functionalization for the preparation of the ion exchanger can be complicated because the dissolved-out polymer fractions accumulate in the reaction solutions used for the functionalization. PA1 3. The end products (i.e., ion exchangers) may contain large amounts of soluble polymer, which may lead to undesired leaching of the ion exchangers. PA1 (a) 34 to 99.7% by weight (preferably 34 to 99.2% by weight, particularly preferably 39 to 99.2% by weight, very particularly preferably 44 to 99.2% by weight, exceedingly preferably 47 to 99.2% by weight) of a monomer having one polymerizable C.dbd.C double bond, PA1 (b) 0.1 to 15% by weight (preferably 0.6 to 15% by weight, particularly preferably 0.6 to 10% by weight, very particularly preferably 0.6 to 5% by weight, exceedingly preferably 0.6 to 2% by weight) of a crosslinking agent having at least 2 (preferably 2 or 3) polymerizable C.dbd.C double bonds, PA1 (c) 0.2 to 1.0% by weight of at least one aliphatic peroxyester as polymerization initiator, and PA1 (d) 0 to 50% by weight (preferably 0 to 40% by weight) of additives, PA1 R.sup.1 denotes an alkyl radical having 2 to 20 carbon atoms or a cycloalkyl radical having up to 20 carbon atoms, PA1 R.sup.2 denotes a branched alkyl radical having 4 to 12 carbon atoms and PA1 L denotes an alkylene radical having 2 to 20 carbon atoms or a cycloalkylene radical having up to 20 C atoms.
It has been found that the polymerization of microencapsulated monomer droplets does not always lead to bead polymers having smooth surfaces. Instead, bead polymers having rough and/or coated surfaces may form. For many applications, surface roughness or a surface coating is not a disadvantage, especially since a surface coating can be removed by an after treatment or can be removed with the capsule wall upon functionalization of the bead polymer to give the ion exchanger. For some applications, however, bead polymers having smooth surfaces are desired, for example, for the use of the bead polymers as seed in a so-called seed/feed process. German Patent Application 19,647,291 states that the addition of water-soluble inhibitors during polymerization of microencapsulated monomer droplets leads to bead polymers having a smooth surface. However, in addition to a smooth surface and a high swelling index, these bead polymers have a soluble fraction that is undesirably high relative to the swelling index.
The object of the present invention is the provision of microencapsulated crosslinked spherical polymers having high surface smoothness, a swelling index between 1.3 and 9, and a content of soluble fractions of less than 6%.
It has been found that polymers having a low content of crosslinking agents and correspondingly high swellability have smooth surfaces and low contents of soluble polymer fractions particularly when, during their preparation, hydroxyaromatic compounds are used as polymerization inhibitors in the aqueous phase and peroxyesters are used as initiators.